Is Quickbooks online down?

Try opening your QBO account on any private/incognito browser.

I appreciate you for reporting to us about the problem, @klec.

Although a cache helps your browser run faster, sometimes it prevents webpages, like in QuickBooks Online, from loading correctly. 

Like what Fiat Lux - Asia suggested, we can start with accessing QuickBooks Online account through your browser's Incognito Mode. Here are the keyboard shortcuts to go to Incognito or InPrivate Mode in your browser:

For Google Chrome / Microsoft Edge / Opera:
 

• Windows: Ctrl + Shift + N
• Mac: Command ⌘ + Shift + N

For Firefox:
 

• Windows: Ctrl + Shift + P
• Mac: Command ⌘ + Shift + P

For Safari:
 

• Shift + Command ⌘ + N
   

If this works, you may proceed to clearing your browser's cache. However, if the error persists, I recommend using another supported browser or device. After you've fixed the error, feel free to read this article about managing your clients in QuickBooks Online: Add and manage customers in QuickBooks Online.

Please don't hesitate to reach us again through the reply button if you have further concerns about QuickBooks Online account and client management. We are always here to assist you.